It's a strap for holding water bottle cap attached to the bottle. To be printed out of flexible material.
Narrow Mouth Nalgene Cap Tether by Inspector5, Download free STL model
1L Nalgene strap with side loop by yanky79, Download free STL model
nalgene 3D Models to Print - yeggi
Nalgene MultiDrink Water Bottle Replacement Cap - Gray
Nalgene 1L bottle cap strap repair by Open Hardware Designs, Download free STL model
Nalgene best 3D printer models・23 designs to download・Cults
nalgene strap 3D Models to Print - yeggi
▷ nalgene bottle cap 3d models 【 STLFinder 】
nalgene strap 3D Models to Print - yeggi
Nalgene 1L bottle cap strap repair by Open Hardware Designs, Download free STL model
Nalgene best 3D printer models・23 designs to download・Cults
▷ nalgene bottle cap 3d models 【 STLFinder 】
Nalgene Bottle Clip by cmcfalls2, Download free STL model